On this week's Weather Wonders episode, Jack asked, "Why do we have different weather changes?". To answer this question, we have to look at high and low pressure systems, as well as, warm and cold fronts and the modifications they make to the airmass around us.
Let's start with high and low pressure. Air flows clockwise around high pressure and counter clockwise around low pressure. The black lines surrounding the high and low pressure systems are isobars or lines of equal pressure. It will increase as you move toward the center of the high pressure and the opposite occurs for the low. When isobars are drawn closer together, that means that the pressure change over an area occurs at a faster rate, compared to further out from the high or low. The change in pressure is the pressure gradient and it resembles the difference in pressure between the high and low. Wind speed is proportional to the change in pressure. When pressure increases or decreases quickly, wind speeds will increase. This explains why we can have windy days, as systems approach or exit Eastern Iowa.
The Pressure Gradient Force, Coriolis Force, and friction all act on wind. These forces cause air to spread out from high pressure systems and move toward low pressure systems. We know air sinks from the higher levels of the atmosphere near high pressure and spreads out along the Earth's surface. This process warms and dries the air as it sinks, allowing evaporation to outperform condensation. High pressure systems are generally associated with warm and dry conditions. Low pressure systems take the air at the surface and force it up high into the atmosphere. This process causes air to cool and condense, which is how we get clouds. Thunderstorms and winter storms are associated with low pressure systems.
Low pressure systems have warm, cold, stationary, and occluded fronts associated with them. They are found at the surface and indicate wind speed, temperature, and dew point changes. Warm fronts represent a warm airmass moving toward a colder one. They are found generally on the east side of low pressure systems.
A cold front shows colder air pushing toward warmer air. They can be found to the south/west of the low pressure system.
Occluded fronts indicate when the cold front has caught up to the warm front. This usually occurs when the low pressure system decreases in strength. Above is a low pressure system plotted, with its associated fronts. Here, you can see wind shifts, along with temperature and dew point changes surrounding the low pressure system. For example, temperatures are warmer and there's a south wind in southern Iowa. Temperatures are in the 30s in Eastern Iowa, with a southwest wind. The warm front will ultimately replace the colder air in Eastern Iowa with the warmer air from the southern part of the state. Near the western part of the state, temperatures are colder with a west wind. The cold front will be moving east and bringing in that colder air.
